1. Field of the Invention
The present invention relates to an optical scanner and an image forming apparatus using the optical scanner. More particularly, the present invention relates to an optical scanner which is suitable for use in an apparatus, such as a laser beam printer or a digital copying machine, making use of an electrophotographic process, and which is used to record image information by deflecting (reflecting) light which has exited from a light source means using a polygon mirror serving as a light deflector, and by scanning a scan surface with the light through a scanning optical means (that is, an fxcex8 lens system). Even more particularly, the present invention relates to an optical scanner which makes it possible to always provide a proper image by correcting the curvature of field and the fxcex8 characteristic in a main scanning direction as a result of forming a plurality of fxcex8 lenses which make up the scanning optical means with proper shapes.
2. Description of the Related Art
Hitherto, in a light scanner of, for example, a laser beam printer, light from a light source means which has been modulated in accordance with an image signal is periodically deflected by a deflecting means, such as a rotating polygon mirror. Then, the deflected light is focused in the form of a spot on the surface of a photosensitive recording medium (that is, a photosensitive drum) by a scanning optical means having an fxcex8 characteristic in order to scan the surface of the recording medium with the light, whereby an image is recorded.
FIG. 22 is a schematic view of the main portion of a conventional light scanner. In FIG. 22, divergent light beams which have exited from a light source means 91 are converted into substantially parallel light beams by a collimator lens 92. These substantially parallel light beams travel to a diaphragm 93 which limits the amount of light incident upon a cylindrical lens 94 having a refractive power only in a subscanning direction. Of the substantially parallel light beams incident upon the cylindrical lens 94, those within a main scanning cross-sectional plane exit from the cylindrical lens 94 as substantially parallel light beams, while those within the subscanning cross-sectional plane are focused in order to form an image which is a substantially linear image on a deflecting surface 95a of a deflecting means 95, which is a rotating polygon mirror.
The light beams which have been deflected (reflected) at the deflecting surface 95a of the deflecting means 95 are led to a photosensitive drum surface 98 serving as a scan surface through a scanning optical means 99 having an fxcex8 characteristic. By rotating the deflecting means 95 in the direction of arrow A, the photosensitive drum surface 98 is scanned with the light beams in order to record image information.
In order to record image information with high precision using this type of light scanner, it is important that the diameter of the spot be the same by properly correcting the curvature of field throughout the entire scan surface, and that the scanning speed on the scan surface be kept the same (fxcex8 characteristic). Conventionally, various light scanners and fxcex8 lens systems (that is, scanning optical means) which satisfy such optical characteristics have been proposed. Since laser beam printers, digital copying machines, and the like are becoming more compact and lower in cost, there has been a demand for making light scanners more compact and lower in cost.
A light scanner which is both more compact and lower in cost is disclosed in, for example, Japanese Patent Laid-Open No. 10-23246. According to this document, curvature of field and distortion are properly corrected, and the influences of, for example, changes in spot diameters due to image heights are made small.
In general, in order to make a light scanner more compact, it is necessary to decrease the focal length of the fxcex8 system, to increase the angle of view, and to bring fxcex8 lenses of the fxcex8 lens system closer to the polygon mirror serving as a deflecting means. All of these make it difficult to correct aberrations, resulting in the problem that the curvature of field and the fxcex8 characteristic in a wide angle-of-view area are not properly corrected when the light scanner is made more compact by the aforementioned ways.
When the angle of view is increased, another problem arises. In conventional light scanners, light which has exited from the light source means is incident upon a deflecting surface of the polygon mirror obliquely from the optical axis of the fxcex8 lens system. The location where the light is deflected (reflected) at the deflecting surface changes continuously and asymmetrically with respect to the center of scanning. This asymmetrical change in the location of deflection influences the location of image formation, making it difficult to obtain a flat curvature of field.
In the first place, the asymmetrical change in the location of reflection occurs because light from the light source means is incident upon the polygon mirror obliquely from the optical axis of the fxcex8 lens system. Therefore, the asymmetrical change can be eliminated by causing the light from the light source means to be incident upon the polygon mirror from the direction of the optical axis of the fxcex8 lens system. However, this is not possible because of the lens arrangement. Since the light must be made incident upon the polygon mirror from outside the fxcex8 lens system, it is impossible to eliminate the asymmetry of the curvature of field caused by asymmetrical changes in the location of reflection.
Examples that make the upper and lower portions of the generating-line shapes of the fxcex8 lenses asymmetrical are proposed in, for example, Japanese Patent Laid-Open Nos. 4-60608 and 9-265041.
In order to make the fxcex8 lens system more compact, it is necessary to properly correct the curvature of field and the fxcex8 characteristic even in an area having a wide angle of view of xc2x147 degrees. Therefore, past proposals have not necessarily made it possible to satisfactorily correct the curvature of field and the fxcex8 characteristic.
In order to make the light scanner capable of using multiple beams, it is necessary to convert the light beams which have exited from the collimator lens into substantially parallel light beams to decrease jitters in the main scanning direction.
Accordingly, it is an object of the present invention to provide an optical scanner which can properly correct curvature of field and distortion in a main scanning direction and curvature of field and changes in magnification in a subscanning direction by forming a plurality of fxcex8 lenses of a scanning optical means with proper shapes, and which is suitable for high-definition printing using a compact structure. It is also an object of the present invention to provide an image forming apparatus using the same.
To these ends, according to a first aspect of the present invention, there is provided a light scanner comprising light-incident optical means that guides light which has exited from light source means to deflecting means, and scanning optical means for forming an image on a scan surface using the light reflected and deflected by the deflecting means. In the light scanner, the scanning optical means comprises at least a first lens and a second lens. The first lens is a meniscus positive lens whose concave surface faces the deflecting means within a main scanning cross-sectional plane, and the second lens is a meniscus lens whose convex surface faces the deflecting means within the main scanning cross-sectional plane. The shape of a light-incident surface of the second lens within the main scanning cross-sectional plane is such that the curvature changes from convex to concave from the optical axis of the second lens to either end of an effective portion of the second lens. Portions of a surface of the second lens at either end of the effective portion are disposed closer to the deflecting means than a portion of the surface of the second lens at an apex thereof on the optical axis of the second lens.
In one form of the first aspect, the first and second lenses are both aspherical lenses, with a light-incident surface and a light-exiting surface of the first lens being displaced towards the scan surface with respect to a base spherical surface, and the light-incident surface and a light-exiting surface of the second lens being displaced towards the deflecting means with respect to a base spherical surface.
In another form of the first aspect, the shapes of a light-incident surface and a light-exiting surface of the first lens and the shape of a light-exiting surface of the second lens within the main scanning cross-sectional plane are such that either end of each surface of the first lens and either end of the surface of the second lens are disposed closer to the deflecting means than each surface of the first lens at an apex of the first lens on the optical axis thereof and the surface of the second lens at an apex of the second lens on the optical axis thereof, respectively. In addition, the curvatures at either end of the light-incident surface and the light-exiting surface of the first lens and the curvatures of the light-incident surface and the light-exiting surface of the second lens are negative.
In still another form of the first aspect, a light-incident surface and a light-exiting surface of the first lens have curvatures that increase from the optical axis of the first lens to either end of the first lens.
In still another form of the first aspect, when the refractive powers of the first and second lenses within the main scanning sectional plane are xcfx861 and xcfx862, respectively, the following condition is satisfied:
xcfx862xe2x89xa6xcfx861/5. 
In still another form of the first aspect, the principal plane of the second lens is disposed closer to the deflecting means than the principal plane of the first lens.
In still another form of the first aspect, when the distance from a deflecting surface of the deflecting means to a surface of the second lens disposed on the side of the scan surface is d, and the effective scanning width on the scan surface is W, the following condition is satisfied:
d/Wxe2x89xa60.2. 
In still another form of the first aspect, when the fxcex8 coefficient of the scanning optical means is k, and the effective scanning width on the scan surface is W, the following condition is satisfied:
k/Wxe2x89xa60.6. 
In still another form of the first aspect, the light source means comprises a plurality of light-emitting sections.
According to a second aspect of the present invention, there is provided an image forming apparatus comprising any one of the above-described forms of the light scanners of the first aspect, and a printer controller for converting code data input from an external device into an image signal, and inputting the image signal to the light scanner.
According to a third aspect of the present invention, there is provided a light scanner comprising light-incident optical means for guiding light which has exited from light source means to deflecting means, and scanning optical means for forming an image on a scan surface using the light deflected by the deflecting means. In the light scanner, the light-incident optical means causes the light which has exited from the light source means to be incident upon a deflecting surface of the deflecting means obliquely from the optical axis of at least one of a plurality of lenses comprising the scanning optical means, the optical axis being within a main scanning cross-sectional plane. At least one of the plurality of lenses has at least one aspherical surface whose shape changes asymmetrically on either side of the optical axis of the at least one lens in a main scanning direction. The at least one aspherical surface of the at least one lens is a tilted generating-line asymmetrical surface wherein a light-source-means end of the surface is closer to the scan surface than the other end of the surface disposed away from the light source means with respect to the optical axis of the at least one lens.
In one form of the third aspect, the tilted generating-line asymmetrical surface is formed on a lens of the plurality of lenses of the scanning optical means disposed closest to the scan surface.
In another form of the third aspect, the light scanner is a multiple-beam light scanner, and the tilted generating-line asymmetrical surface is formed on one or both surfaces of the at least one of the plurality of lenses of the scanning optical means.
In still another form of the third aspect, the at least one aspherical surface of the at least one of the plurality of lenses is a generating-line asymmetrical curvature surface such that the surface has at the light-source-means end a curvature greater than the curvature at the other end of the lens effective portion disposed away from the light source means with respect to the optical axis of the at least one lens.
When the at least one aspherical surface of the at least one of the plurality of lenses is a generating-line asymmetrical curvature surface such that the surface has at the light-source-means end a curvature greater than the curvature at the other end of the lens effective portion disposed away from the light source means with respect to the optical axis of the at least one lens, the generating-line asymmetrical curvature surface may be formed on a lens of the plurality of lenses of the scanning optical means disposed closest to the scan surface.
When the at least one aspherical surface of the at least one of the plurality of lenses is a generating-line asymmetrical curvature surface such that the surface has at the light-source-means end a curvature greater than the curvature at the other end of the surface disposed away from the light source means with respect to the optical axis of the at least one lens, the generating-line asymmetrical curvature surface may be formed on one or both surfaces of the at least one of the plurality of lenses of the scanning optical means.
According to still another form of the third aspect, the scanning optical means comprises first and second lenses, and, when the refractive powers of the first and second lenses within the main scanning cross-sectional plane are xcfx861 and xcfx862, respectively, the following condition is satisfied:
xcfx862xe2x89xa6xcfx861/5. 
According to still another form of the third aspect, the principal plane of the second lens is disposed closer to the deflecting means than the first lens.
According to still another form of the third aspect, when the distance from the deflecting surface of the deflecting means to a surface of the second lens disposed at the side of the scan surface is d, and the effective scanning width on the scan surface is W, the following condition is satisfied:
d/Wxe2x89xa60.2. 
According to still another form of the third aspect, the light-source-means side of an effective portion of each of first and second lenses is longer than the side of the effective portion of each of the first and second lenses disposed away from the light source means with respect to the optical axis of each of the first and second lenses.
According to still another form of the third aspect, when the fxcex8 coefficient of the scanning optical means is k, and the effective scanning width on the scan surface is W, the following condition is satisfied:
k/Wxe2x89xa60.6. 
According to still another form of the third aspect, the light-incident optical means comprises a condenser lens for converting the light which has exited from the light source means into either substantially parallel light beams or weakly focused light beams, and, when the distance from the deflecting surface of the deflecting means to a point where the light beams are naturally focused by the condenser lens is L, and the fxcex8 coefficient of the scanning optical means is k, the following condition is satisfied:
|L|xe2x89xa73xc3x97k. 
According to still another aspect of the third aspect, the light source means comprises a plurality of light-emitting sections.
According to a fourth aspect of the present invention, there is provided an image forming apparatus comprising any one of the above-described the light scanners of the third aspect, and a printer controller for converting code data input from an external device into an image signal, and inputting the image signal to the light scanner.
According to a fifth aspect of the present invention, there is provided a light scanner comprising light-incident optical means for guiding light which has exited from light source means to deflecting means, and scanning optical means for forming an image on a scan surface using the light reflected and deflected by the deflecting means. In the light scanner, the light-incident optical means causes the light which has exited from the light source means to be incident upon a deflecting surface of the deflecting means obliquely from the optical axis of the scanning optical means within a main scanning cross-sectional plane. The scanning optical means comprises at least a plurality of lenses, with at least one of the plurality of lenses being within the main scanning cross-sectional plane and having at least one aspherical surface whose shape changes asymmetrically on either side of the optical axis of the at least one lens in a main scanning direction. The at least one aspherical surface of the at least one of the plurality of lenses is a generating-line asymmetrical curvature surface such that the surface has at a light-source-means end a curvature greater than at the other end of the surface disposed away from the light source means with respect to the optical axis of the at least one lens.
In one form of the fifth aspect, the generating-line asymmetrical curvature surface is formed on the lens of the plurality of lenses of the scanning optical means disposed closest to the scan surface.
In another form of the fifth aspect, the generating-line asymmetrical curvature surface is formed on one or both surfaces of the at least one of the plurality of lenses of the scanning optical means.
In still another form of the fifth aspect, the scanning optical means comprises first and second lenses, and, when the refractive powers of the first and second lenses within the main scanning cross-sectional plane are xcfx861 and xcfx862, respectively, the following condition is satisfied:
xcfx862xe2x89xa6xcfx861/5. 
In still another form of the fifth aspect, the principal plane of the second lens is disposed closer to the deflecting means than the first lens.
In still another form of the fifth aspect, when the distance from the deflecting surface of the deflecting means to a surface of the second lens disposed at the side of the scan surface is d, and the effective scanning width on the scan surface is W, the following condition is satisfied:
d/Wxe2x89xa60.2. 
In still another form of the fifth aspect, the light-source-means side of an effective portion of each of first and second lenses is longer than the side of the effective portion of each of the first and second lenses disposed away from the light source means with respect to the optical axis of each of the first and second lenses.
In still another form of the fifth aspect, when the fxcex8 coefficient of the scanning optical means is k, and the effective scanning width on the scan surface is W, the following condition is satisfied:
k/Wxe2x89xa60.6. 
In still another form of the fifth aspect, the light-incident optical means comprises a condenser lens for converting the light which has exited from the light source means into either substantially parallel light beams or weakly focused light beams, and, when the distance from the deflecting surface of the deflecting means to a point where the light beams are naturally focused by the condenser lens is L, and the fxcex8 coefficient of the scanning optical means is k, the following condition is satisfied:
|L|xe2x89xa73xc3x97k. 
In still another form of the fifth aspect, the light source means comprises a plurality of light-emitting sections.
According to a sixth aspect of the present invention, there is provided an image forming apparatus comprising any one of the forms of the light scanners of the fifth aspect, and a printer controller for converting code data input from an external device into an image signal, and inputting the image signal to the light scanner.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.